Study On The Chemical Composition And The Surface Microbial Diversity For Nuo Deng Ham

Nuo Deng ham is a specialty of Nuodeng Village,Yunlong County,Dali Prefecture,Yunnan Province.Because of its unique production process,high quality and delicious taste,Nuo Deng ham was registered in the National Trademark Office in 2004 and became international famous brand product with a history over 1,000 years and a modern green safe food concept.In the process of ham fermentation,a series of biochemical reactions occur.As the fermentation time prolongs,the physical and chemical indicators change,and these changes will affect the quality of final product.Microbial activity is an important reason for changes in physical and chemical indicators of ham.Therefore,it is of great significance to analyze the changes of physicochemical indexes and surface microbial diversity during the ham fermentation.Currently,the research on ham in the domestic and foreign mainly focused on Jin Hua ham,Xuan Wei ham and Western-style ham.There is no systematic study on Nuo Deng ham.The 1⁴ ages of Nuo Deng Ham^TMM was used as the test material,and the physicochemical indexes and volatile compounds of different ages of Ham^TM were analyzed by national standard method and GC-MS,The surface of different ages of ham's microbial diversity were initially studied by cultivation method and high-throughput sequencing,the main results are summarized below:The moisture content of Ham^TM decreased year by year,while the pH range maintained from 5.90 to 6.05,The amino acid content was the highest in the three-year ham,which was 7.23%,The salt content of the Ham^TM was the highest?6.82%?for two years.Nitrate content increased year by year,The content of nitrite was 0.20 mg/kg for one year,while was not detected in ham for two years or three years,The highest level in four years was 0.56 mg/kg,which was far lower than the residue of food additives used in China,The residue is less than or equal to 70 mg/kg.In addition,It was found that the chemical composition of ham in one year was different from that of other years,followed by two years,and the difference between three years and four years is minimal.The study found that the main volatile flavor compounds of Nuo Deng ham were aldehydes,esters,alcohols,hydrocarbons,ketones,carboxylic acids,furan,pyrazine and naphthalene,etc.Among them,the type and content of aldehydes and esters were the highest percents,which played a major role in the formation of flavor of Nuo Deng ham.2,4-Decadienal was the highest content of aldehydes,Which was also present in other hams.As a whole,the main volatile components of Nuo Deng ham were the highest in three years of fermentation.The results of colony counting showed that the surface flora microorganisms of Nuo Deng ham were very rich,but the bacteria group was far more than fungi.The abundances of bacteria and fungi are the highest on the surface of three-year-aged ham.The predominant group,Which was isolated from mold is Aspergillus,while the identified yeasts are mainly Debaryomyces and Candida,Staphylococcus is a dominant genus in culturable bacteria,which plays an important role in the ham's fermentation process.In addition,Bacillus strains were isolated.The results of fungal high-throughput sequencing based on HiSeq showed that the number of OTU was the highest in three years and present in the surface of Nuo Deng ham,and the predominant fungal groups were Ascomycota and Basidiomycota.The top10 genera were Cercospora,Candida,Wallemia,Aspergillus,Phialosimplex,Aureobasidium,Pithoascus,Xerochrysium,Scopulariopsis and Cryptococcus.The main dominant fungal populations,identified at the genus level,were Cercospora,Candida,Wallemia,and Aspergillus.There were significant difference in the fungal community structure and the species diversity present in the surface of Nuo Deng ham among different years.The fungal community richness was ND3>ND4>ND2>ND1?ND stands for Nuo Deng ham and figures represent different years?.In the class of Saccharomycetes,Candida had the highest abundance during the one and two years ham,while Aspergillus is the highest in ham after three and four years.It can be seen that yeast may plays a major role in the early stage of ham fermentation,while Aspergillus plays a major role in the late fermentation period.The phylogenetic tree at the genus level indicates that the main mold on the surface of the ham is Aspergillus.In addition,there was a correlation between all physicochemical factors and fungal communities.Compared with other factors,salt content,nitrate and nitrite content were highly correlated with fungal communities.Bacterial high-throughput sequencing results showed that the surface bacterial and fungal populations were similar in Nuo Deng ham in different fermentation periods,and the number of OTU was the highest in three years.The dominant bacterial populations at the phylum level are Firmicutes,Proteobacteria,Bacteroidetes,Cyanobacteria,and Actinomycetes,while the dominant bacterial populations at the genus level are Staphylococcus,Psychrobacter,Solitalea and Shigella.The Staphylococcus has a high abundance in one and two year fermentation of ham,suggesting that Staphylococcus may play a major role in the early stage of ham fermentation.Through the Chao1 index and the Shannon index,it was found that the bacterial community richness was ND3>ND2>ND4>ND1 and the community diversity was ND3>ND4>ND1>ND2 in the different ages of ham.At the same time,the experimental results also showed that the three ages of ham had a different bacterial community structure in several other years.There was a correlation between the physicochemical factors and the bacterial community.Compared with other factors,the salt content,nitrate and amino acid content were significantly correlated with the bacterial community.The results of both cultivation method and high-throughput sequencing showed that the surface microbial abundance of Nuo Deng ham was the highest after three years of fermentation.The abundance and diversity of microorganisms identified in the high-throughput sequencing was much higher than that of cultivation method,Therefore,High-throughput sequencing could response to the original microbial community structure more reasonable and more complete.This study analyzed the changes in physicochemical index of Nuo Deng ham at different fermentation periods.At the same time,the microbial diversity in the surface of Nuo Deng ham was analyzed,and a large number of microbial strains were isolated.This information can provide the strain resources for the development of meat products with excellent properties,and it has theoretical significance for the improving of the fermentation process of Nuo Deng ham.